For many precision engineering systems, it is required to measure the position of an object to nm-level tolerances over long periods of time, e.g., days, months, or even years. The range of motion of the object may be small, e.g., less than a mm in any direction, but should be controlled with an active servo loop for stability and to provide for fine, controlled adjustments.
An example of such a precision engineering system is the projection objective (PO) assembly of a modern photolithography tool. Examples of PO assemblies are described in Microlithography: Science and Technology, edited by J. R. Sheats and B. W. Smith, Marcel Decker, Inc. (New York, N.Y. 1998). PO assemblies include dioptric PO assemblies, catadioptric PO assemblies, and catoptric PO assemblies.
In such PO assemblies, various subassemblies comprised of lenses are held in relationship to each other and/or in relation to a PO assembly lens cell by means of small actuators that operate in conjunction with sensors placed throughout the PO assembly which is comprised of the lens elements and the cell to measure relative positions. These sensors may be, for example, encoder devices as described by A. H. Slocum in Precision Machine Design, Englewood Cliffs, N.J.: Prentice Hall, pp. 162-173. (1992), and are used for position feedback control.